I found a 2011 slide show with innovative PLCs from German book publisher Vogel. They asked some companies to search their archives about the beginnings of PLC. Most of the companies still exist, some were aquired by ABB. Most of them I never heard of. Also a comparison of old and (then) new. For some I even found earlier ones.
First of interest B&R Multicontrol (1983). First one with parallel CPUs for analog and digital processing (6303+6809). I found some manuals. Nice design, black front with yellow/orange connectors. (Slide 8) PILZ (slide 9) also has yellow. Programming was in the usual different ways incl ladder logic. Programming via statement list could be in either B&R specific, German command abbreviations or even original Motorola mnemonics.
Most of interest is HIMA (slides 17-19) and their Planar F (since 1970, not 1979). Failsafe security systems. They claim to be the only one with the highest security level SIL 4. But I read about higher standards up to 7 for space and nuclear. I found some short manuals and brochures but none about detailed programming including very few info about Planar F. One card used an 8052. A later card used 2x 386. Most or all newer ones are SIL 3.
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On ebay I found Swedish SATT (SattCon/SattControl since 1983), originally Electronlund, now ABB. Since 1986 also Alfa-Laval (who had cream separators) and was aquired by Tetra Pak. Early PLCs were PBS and SattCon05. Software was DOX5 and DOX10. I found some few manuals, mainly officially on ABB. The addressing of digital signals was in octal. Some early systems used an 6802. SC05 an 80C320. Slim cards with silver front.
Another interesting vintage system was Kuhnke. I found several vintage OOP manuals (incl programming) here